The present invention relates to a frequency-division-multiplexing (FDM)-time-division-multiplexing (TDM) transmultiplexing system applicable to a regenerative repeating system which is associated with a satellite or a ground radio communication system.
In parallel with the advance of digital communication technologies, the demand for an FDM-TDM transmultiplexer which interconnects the conventional FDM analog modulated signal system and the TDM digital signals system is increasing. As regards satellite communications, there is an increasing demand for a modulation/demodulation device capable of performing FDM-TDM conversion on board a satellite in order to transmultiplex traditional single channel per carrier (SCPC) signals and modern TDMA signals. Further, because the data rate becomes diversified with the evolution of mobile communication and business communication, a group modulation/demodulation device to be loaded has to meet requirements not only in the aspect of performance but also in that of flexibility of functions.
Today, a Fast Fourier Transform (FFT)-filter method and a chirp-z-transform method are available as FDM-TDM transmultiplexing implementations. In the chirp-z-transform method, a chirp filter and a chirp signal generator have heretofore been implemented with Surface Acoustic Wave (SAW) elements. This is advantageous, however, since the delay time which is attainable with SAW elements is limited and, therefore, it is difficult for FDM-TDM translation to be achieved on slow signals, i.e. signals in a narrow frequency band. Concerning matching between the chirp signal generator and the chirp filter, should it be distorted, time separation between channels would be effected to bring about crosstalk between the channels. This would result in fluctuations in the characteristics of the SAW elements due to temperature as well as in long-term fluctations, deteriorating characteristics as a whole. Especially, a transmultiplexer needs an utmost stability since it is expected to process a great number of signals collectively. Concerning business communication, on the other hand, while a plurality of data rates have to be accommodated, the chance for the method using SAW elements to successfully realize a required degree of flexibility is scarce.
Meanwhile, the FFT-filter method is implemented with digital circuits only and, therefore, very stable in characteristics. Although the FFT-filter method has been extensively applied to a transmultiplexer for the above reason, for N channels it requires an N-point FFT circuit and N digital subfilters with the result that the circuit is scaled up in proportion to the number of channels N.